Patent-Related Reference 1: Japanese Patent Application No. 2003-119000
Patent-related Reference 2: JP-A-10-265293
Patent-related Reference 3: JP-A-8-259375
Patent-unrelated reference 1: Applied Physics Handbook, second edition, p427, MARUZEN Co., Ltd.
Highly integrated semiconductor devices have brought about various light sources at shorter wavelengths, and the needs thereof have reached a vacuum ultraviolet range. Utilized as an optical material in this wavelength range is a fluoride crystal exhibiting an excellent transmissivity, and a single crystal of fluoride such as calcium fluoride is used as an optical material for optical lithography where ArF excimer laser (193 nm) and F2 excimer laser (157 nm) are used, for example. Additionally, it is earnestly desired to develop a novel fluoride crystal usable as: a crystal for all-solid-state ultraviolet/infrared laser; a window material for an ultraviolet range; an optical material for a medical treatment; and the like.
These fluoride crystals are each grown as a bulk single crystal mainly by a Bridgman method, a Cz method, or the like, thereafter worked by cutting, and used for various usage and for measurement. However, such methods require enormous costs and a number of days to obtain a single crystal, thereby bringing about an obstruction to a development speed of a novel material.
Meanwhile, concerning production of a single crystal and eutectic crystal of oxide, and production of Si, there has been known a micro-pulling-down method (patent-related reference 2, patent-related reference 3, and patent-unrelated reference 1). For example, the patent-related reference 2 describes a definite apparatus, at its paragraph numbered (0025) and in its FIG. 1.
In the techniques described in the patent-related references 2, 3, and patent-unrelated reference 1, there is enabled a crystal growth at a rate one order or two orders higher than those of the other melt growth methods. This leads to shorter times to be required for production of crystal, thereby enabling obtainment of a single crystal in a meaningful size with high-quality from a small amount of raw material. It is further possible to grow a crystal without eliminating impurities floating on an upper surface of melt, because the crystal is drawn through a small hole at a bottom of a crucible.
The present inventor has applied the micro-pulling-down method to fluorides, investigated an apparatus and production method therefor (patent-related reference 1), and succeeded in growing a crystal having a diameter on the order of 1 mm. However, it is becoming apparent that the crystal grown by this method is required to have an adapted shape in case of evaluation as to whether or not the crystal is suitable as a material for various usages such as solid-state laser and scintillator.